Thermoplastic rewelding process

ABSTRACT

Thermoplastic welding is an emerging technology targeted at significantly reducing the manufacturing cost of aerospace structure by eliminating fasteners and the touch labor associated with fasteners to prepare, install, and inspect the assemblies. Quality welds are highly dependent upon achieving appropriate temperatures everywhere along the bond line. The present invention is a system that evaluates the quality of the welds involving inputting an EM pulse to the embedded susceptor and listening to the acoustic response that the pulse generates to determine weld quality from the sound.

REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application based upon U.S.patent application Ser. No. 08/907,533, filed Aug. 8, 1997, now U.S.Pat. No. 5,902,935.

The present application claims the benefit of U.S. ProvisionalApplication No. 60/025,343, filed Sep. 3, 1996.

TECHNICAL FIELD

The present invention is a nondestructive method for evaluating thequality and integrity of a thermoplastic weld having an embeddedsusceptor. The method uses an impulse coil to vibrate the susceptor andan acoustic sensor to listen to the vibration to assess the weldquality, generally through analysis of the return signal in thefrequency domain.

BACKGROUND ART

Composite materials lend themselves to bonded structures better than tofastened ones. Bonded composites have received limited use in criticalaerospace structures, however, because the bonds can vary in strength orstiffness even if they have no discrete bond line defects (disbonds,porosity, voids, cracking, etc.). Traditional nondestructive inspectionmethods rely upon quantifying these defects to predict theflightworthiness of the structure, but are unable to ascertain thecohesiveness of the bond at any location if defects are absent.Nondestructive identification of low strength bonds and regions of“kissing unbonds” (bonds of near zero strength) remains a significantgoal solved only in a few specific bonded applications where the resultsof shear or tensile tests have been correlated to a particular NDEsignal feature. Modified pulse-echo ultrasonic testing (UT) has beensuccessful in finding the discrete defects (voids, delaminations,porosity), but not “kissing unbonds” and low strength bonds. Infraredthermography, shearography, eddy current, and various high and lowfrequency ultrasonic methods have also been unsuccessful in discerningbond quality in thermoplastic welds.

The present invention provides a nondestructive method for testing bondquality using an electromagnetic (EM) pulse to induce vibrations in theembedded susceptor and an acoustic receiver to listen to and to recordthe induced vibrations. Analysis of the received vibration signaldiscriminates bond quality.

SUMMARY OF THE INVENTION

The present invention inspects bond lines that contain conductivematerial, especially those formed using a copper mesh susceptor, usinghigh energy electromagnetic pulsing with acoustic receiving in a singleinspection head to produce a unique evaluation technique. The inspectionhead contacts the outer skin of the structure whose bond is beingevaluated, and contains a pancake type copper coil containing windingswith rectangular cross sections designed to effectively couple to thesusceptor. The closing of a switch releases a charge built up in acapacitor bank, which creates a high energy pulse in the coil. Theelectromagnetic field produced by the current pulse couples to thesusceptor, opposes the driving magnetic field of the coil, and creates aforce on the susceptor. With the help of a finite element code forelectromagnetic interactions, we have been able to model the test setupand predict the forces on the susceptor. For the EM pulse produced bythe discharge of a capacitor bank (proportional to the voltage of thecoil), the normal stresses induced in the susceptor are shown in FIG. 3.The frequency of the stress peaks in the time domain is twice that ofthe voltage peaks at the coil, because a change (positive or negative)occurs on both sides of each peak.

The stresses incite vibrational modes in the susceptor and surroundingcomposite, creating an acoustic wave that we receive and record with anelectromagnetically shielded acoustic-emission (AE) transducer at thecenter or around the outside of the coil (FIG. 4). While the size of thesignal will depend upon the size of the incoming pulse, the susceptorconductivity, and the depth of the bond line, frequency-related featuresof the signal can be correlated to bond quality. We have demonstratedexperimentally that a susceptor that is not fully bonded to thesurrounding substructure will produce low frequency modes that areabsent in a well bonded structure. The difference in the frequencyresponse produced from a good bond and a poor bond is shown in FIGS. 5and 6, respectively. FIGS. 5 & 6 plot Fourier transforms from the timedomain to the frequency domain of the ultrasonic signal received at theAE transducer.

Bonds on both sides of the susceptor can be separately examined. Thesebonds experience both tensile and compressive stress when the susceptorvibrates. As the vibration occurs, the bond above the susceptor will beunder tension as the bond below is under compression, and visa versa. Wemeasure the response of the bond to a given induced stress level,permitting a type of in-situ proof-testing of the bond quality. “Kissingunbonds” or low strength bonds can be identified with lower energypulses.

Some radomes contain conductive layers (the FSS layers), which may beinspected with this device. In addition, a conductive layer can be addedto adhesive bond lines to produce an inspectable bonded structure froman otherwise uninspectable one.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a moving coil thermoplastic weldingapparatus.

FIG. 2 is a schematic view, partially in section, illustrating a cupcoil for inducing vibrations in the susceptor of thermoplastic welds.

FIG. 3 is a graph showing the normal stresses induced in a susceptorembedded in a thermoplastic weld by EM pulses from the coil of FIG. 2.

FIG. 4 is a schematic graphical representation of the acoustic signalcreated by pulsing a susceptor in a well bonded thermoplastic weld.

FIG. 5 is a graphical representation of the acoustic signal created bypulsing a susceptor in an adequate strength, good quality thermoplasticweld.

FIG. 6 is a graphical representation of the acoustic signal created bypulsing a susceptor in a low strength, poor quality thermoplastic weld.

FIG. 7 is a schematic plan view of a typical susceptor tape.

FIG. 8 is a schematic representation of the maximum stress on thesusceptor as a function of position from the centerline of the coilassuming there is no offset between the coil and susceptor.

FIGS. 9 A-D are graphs showing the typical correlation between bondstrength and the area under the frequency domain Fourier Transform curvefor a low frequency response for a narrow susceptor.

FIG. 10 is a graph showing relative bond strength as a function of thearea under the low frequency response curve.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

First, we will describe a typical thermoplastic welding operation, sincethese are the bonds of principle interest for nondestructive evaluationin accordance with the present invention. Then, we will describe thenondestructive evaluation system (NDE) of the present invention forassessing bond quality.

For purposes of this description, “laminate” means a fiber-reinforcedorganic resin matrix composite having a plurality of plies of prepreg orits equivalent consolidated together and cured, as appropriate. Thelaminates are prefabricated by any appropriate means including automaticor hand tape lay up or tow fiber placement with autoclave consolidationand cure; resin transfer molding (RTM); SCRIMP; or the like. Generally,the organic matrix resin is a thermoplastic, especially PEK, PEEK, PEKK,ULTEM polyimide, or KIII. In the welding operation, resin in thelaminates as well as resin in the susceptor melts, intermixes, and fusesto form the weld. The laminate might also be a thermoset in which casethe welding process actually forms a hot melt adhesive bond rather thana weld. We prefer welding, but recognize application of our NDE/NDIprocess to assess the strength and quality of adhesive bonds.

In a thermoplastic laminate, the reinforcing fiber typically is carbonfiber in continuous or chopped form, and generally as tow or wovenfabric. While other fibers can be used, modern aerospace requirementsmost often dictate carbon fibers for their strength and durability, andwe prefer them. In thermosets, especially epoxy, the fibers might begraphite or fiberglass.

THERMOPLASTIC WELDING

Three major joining technologies exist for joining aerospace compositestructure: mechanical fastening; adhesive bonding; and welding. Bothmechanical fastening and adhesive bonding use costly, timeconsumingassembly steps that introduce excess cost into the manufacture ofaerospace composite assemblies even if the parts are fabricated fromcomponents produced by an emerging, cost efficient process. Mechanicalfastening requires expensive hole locating, drilling, shimming, andfastener installation, while adhesive bonding usually requirescomplicated surface pretreatments.

In contrast, composite welding eliminates fasteners and can jointhermoplastic composite components at high speeds with minimum touchlabor and little, if any, pretreatments. In our experience, the weldinginterlayer, called a susceptor, also can simultaneously take the placeof shims required in mechanical fastening. As such, composite weldingholds promise to be an affordable joining process. For “welding”thermoplastic and thermoset composite parts together, the resin that thesusceptor melts functions as a hot melt adhesive. If fully realized,this thermoplastic-thermoset bonding process, in addition to truethermoplastic welding, will further reduce the cost of compositeassembly.

Thermoplastic welding is a process for forming a fusion bond between thefaying thermoplastic faces of two or more parts. A fusion bond iscreated when the thermoplastic on the surface of the two parts is heatedto the melting or softening point and the two surfaces are brought intocontact so that the molten thermoplastic mixes. Then, the surfaces areheld in contact while the thermoplastic cools below the softeningtemperature to fuse the thermoplastic into the weld.

There is a significant stake in developing a successful thermoplasticwelding process. Its advantages versus traditional composite joiningmethods are:

reduced parts count versus fasteners

minimal surface preparation, in most cases a simple solvent wipe toremove surface contaminants

indefinite shelf life at room temperature

short process cycle time, typically measured in minutes when usinginduction heating

enhanced joint performance, especially hot/wet and fatigue

permits rapid field repair of composites or other structures.

little or no loss of bond strength after prolonged exposure toenvironmental influences.

The exponential decay of the strength of magnetic fields with distancefrom their source dictates that, in induction welding processes, thestructure closest to the induction coil will be the hottest, since itexperiences the strongest field. Therefore, it is difficult to obtainadequate heating at the bond line between two graphite or carbon fiberreinforced resin matrix composites relying on the susceptibility of thefibers alone as the source of heating in the assembly. For the innerplies to be hot enough to melt the resin, the outer plies closer to theinduction coil and in the stronger magnetic field are too hot. Thematrix resin in the entire piece of composite melts. The overheatingresults in porosity in the product, delamination, and, in some cases,destruction or denaturing of the resin. To avoid overheating of theouter plies and to insure adequate heating of the inner plies, asusceptor of significantly higher conductivity than the fibers is usedto peak the heating selectively at the bond line of the plies whenheating from one side. An electromagnetic induction coil on one side ofthe assembly heats a susceptor to melt and cure a thermoplastic resin(also sometimes referred to as an adhesive) to bond the elements of theassembly together. Often the current density in the susceptor is higherat the edges of the susceptor than in the center because of thenonlinearity of the coil. This problem typically occurs when using a cupcore induction coil like that described in U.S. Pat. No. 5,313,037 andcan result in overheating the edges of the assembly or underheating thecenter, either condition leading to inferior welds because ofnon-uniform curing. It is necessary to have an open or mesh pattern inthe susceptor to allow the resin to bond between the composite elementsof the assembly when the resin heats and melts. Misalignment can alsoresult in temperature variations, producing excessive heating inisolated locations because of the induction physics. U.S. patentapplication Ser. No. 08/565,566 describes one mechanism for achievingproper alignment between the moving induction coil and the susceptor toreduce problems associated with excessive heating.

U.S. Pat. No. 4,673,450 describes a method to spot weld graphite fiberreinforced PEEK composites using a pair of electrodes After rougheningthe surfaces of the prefabricated PEEK composites in the region of thebond, Burke placed a PEEK adhesive ply along the bond line, applied apressure of about 50-100 psi through the electrodes, and heated theembedded graphite fibers by applying a voltage in the range of 20-40volts at 30-40 amps for approximately 5-10 seconds with the electrodes.Access to both sides of the assembly is required in this process whichlimits its application.

Prior art disclosing thermoplastic welding with induction heating isillustrated by U.S. Pat. Nos. 3,966,402 and 4,120,712. In these patents,the metallic susceptors are of a conventional type having a regularpattern of openings of traditional manufacture. Achieving a uniform,controllable temperature in the bond line, which is crucial to preparinga thermoplastic weld of adequate integrity to permit use of welding inaerospace primary structure, but is difficult to achieve with thoseconventional susceptors, as we discussed and illustrated in U.S. Pat.No. 5,500,511.

Simple as the thermoplastic welding process sounds, and as easy as it isto perform in the laboratory on small pieces, it becomes difficult toperform reliably and repeatably in a real factory on full-scale parts tobuild a large structure such as an airplane wing box. One difficulty isin getting the proper amount of heat to the bond line withoutoverheating the entire structure. Another is achieving intimate contactof the faying surfaces of the two parts at the bond line during heatingand cooling despite the normal imperfections in the flatness ofcomposite parts, thermal expansion of the thermoplastic during heatingto the softening or melting temperature, flow of the thermoplastic outof the bond line under pressure, and then contraction of thethermoplastic in the bond line during cooling.

a. Moving Coil Welding Processes

In U.S. Pat. No. 5,500,511, Boeing described a tailored susceptor forapproaching the desired temperature uniformity. This susceptor reliedupon carefully controlling the geometry of openings in the susceptor(both their orientation and their spacing) to distribute the heatevenly. For example, using a regular array of anisotropic, diamondshaped openings with a ratio of the length (L) to the width (W) greaterthan 1 provided a superior weld over that achieved using a susceptorhaving a similar array, but one where the L/W ratio was one. By changingthe length to width ratio (the aspect ratio) of the diamond-shapedopenings in the susceptor, Boeing achieved a large difference in thelongitudinal and transverse conductivity in the susceptor, and, thereby,tailored the current density within the susceptor. A tailored susceptorhaving openings with a length (L) to width (W) ratio of 2:1 has alongitudinal conductivity about four times the transverse conductivity.In addition to tailoring the shape.of the openings to tailor thesusceptor, Boeing altered the current density in regions near the edgesby increasing the foil density (i.e., the absolute amount of metal).Increasing the foil density along the edge of the susceptor increasedthe conductivity along the edge and reduced the current density and theedge heating. The tailored susceptor had increased foil density byfolding the susceptor to form edge strips of double thickness or bycompressing openings near the edge of an otherwise uniform susceptor.Boeing found this susceptor difficult to reproduce reliably. Also, itsuse forced careful placement and alignment to achieve the desired effectwhen using the cup coil of U.S. Pat. No. 5,313,037 and the multipasswelding process of U.S. Pat. No. 5,486,684, both of which we incorporateby reference.

With the cup coil, the magnetic field is strongest near the edgesbecause the central pole creates a null at the center. Therefore, thesusceptor is designed to counter the higher field at the edges byaccommodating the naturally higher induced current near the edges. Thehigh longitudinal conductivity encourages induced currents to flowlongitudinally.

With the tailored susceptor or with other moving coil weldingoperations, achieving the proper bond line temperature requiresempirical design calibration. Even then, the bond line temperature mayfluctuate within a relatively wide range because of misalignment,variations in the susceptor, variations in the geometry (such as skinplies or spar curvature), or variations in coil speed or coil power.Boeing has created calibration curves (i.e., allowables data) for aspecified power at a specified head speed, geometry, and materialsystem. The allowables data must be quite extensive, and there is stillno assurance that an actual run is producing a weld that corresponds tothe test data. Monitoring the bond line temperature in real time toachieve uniform temperatures at the bond line has great significance toachieving process control and quality welds. Assessing the weld qualitynondestructively is also essential since bonds of inadequate strengthwould produce catastrophe for the end product.

Boeing described a selvaged susceptor for thermoplastic welding in U.S.Pat. No. 5,508,496. That selvaged susceptor controls the current densitypattern during eddy current heating by an induction coil to providesubstantially uniform heating to a composite assembly and to insure thestrength and integrity of the weld in the completed part. This susceptoris particularly desirable for welding ribs between prior welded sparsusing an asymmetric induction coil of U.S. Pat. No. 5,444,220, becausethat coil provides a controllable area of intense, uniform heating underthe poles, a trailing region with essentially no heating, and a leadingregion with minor preheating. We incorporate these patents by reference.

Boeing achieved better performance (i.e., more uniform heating) in ribwelding by using the selvaged susceptor having a center portion with aregular pattern of openings and solid foil edges, which it refers to asselvage edge strips. Embedding the susceptor in a thermoplastic resinmakes a susceptor/resin tape that is easy to handle and to use inassembling the composite pieces prior to welding. With a selvagedsusceptor, the impedance of the central portion should be anisotropicwith a lower transverse impedance than the longitudinal impedance. Here,the L/W ratio of diamond shaped openings should be less than or equal toone. That is, unlike the tailored susceptor of U.S. Pat. No. 5,500,511,L for the selvaged susceptor of U.S. Pat. No. 5,508,496 should be lessthan W. With this selvaged susceptor, in the region immediately underthe asymmetric induction work coil described in U.S. Pat. No. 5,444,220,current flows across the susceptor to the edges where the currentdensity is lowest and the conductivity, highest.

Generally, the selvaged susceptor is somewhat wider than the bond lineso that the selvage edge strips extend on either side of the bond line.Removal of the selvage edge strips after forming the weld leaves only aperforated susceptor foil in the weld. This foil has a relatively highopen area fraction.

A structural susceptor allows Boeing to include fiber reinforcementwithin the weld resin to alleviate residual tensile strain otherwisepresent in an unreinforced weld. The susceptor includes alternatinglayers of thin film thermoplastic resin sheets and fiber reinforcement(usually woven fiberglass fiber) sandwiching the conventional metalsusceptor that is embedded in the resin, and is described in greaterdetail in U.S. patent application Ser. No. 08/471,625. While the numberof total plies in this structural susceptor is usually not critical,Boeing prefers to use at least two plies of fiber reinforcement on eachside of the susceptor.

The structural susceptor permits gap filling between the weldedcomposite laminates which tailors the thickness (number of plies) in thestructural susceptor to fill the gaps, thereby eliminating costlyprofilometry of the faying surfaces and the inherent associated problemof resin depletion at the faying surfaces caused by machining thesurfaces to have complementary contours. Standard manufacturingtolerances produce gaps as large as 0.120 inch, which is too wide tocreate a quality weld using the conventional susceptors.

Boeing can easily tailor the thickness of the structural susceptor tomatch the measured gap by scoring through the appropriate number ofplies of resin and fiber reinforcement and peeling them off. In doingso, a resin rich layer will be on both faying surfaces and this layershould insure better performance from the weld.

To form a structural susceptor, Boeing takes a barbed susceptor andloosely bonds fiberglass reinforcing fiber and thermoplastic films inalternating layers symmetrically on both sides, similar to what is shownin U.S. patent application Ser. No. 08/471,625. The fiberglassreinforcement prevents the resin from fracture under the residual strainleft after welding. Higher ductility resins such as PEEK, PEK, and ULTEMpolyimide also resist fracture better than some thermoplastics. Thethermoplastic films are preferably the same resin as that used to embedthe metal foil and to fabricate the laminates. Sheet thicknesses forthese films are usually about 0.001-0.002 inch (0.025-0.050 mm). Thewoven fibers are preferably oriented perpendicular and parallel to thelongitudinal axis of the weld.

The structural susceptor is generally loosely bonded together by heat orpressure or both, but could be of essentially unitary construction ifdesired. Being loosely bonded helps in gap filling. Boeing uses at leasttwo layers of fiber and thermoplastic on each side of the susceptor, butthe absolute number is not critical. Boeing tested four different stylesof fiberglass and achieved similar results with each, so the type orstyle of fiberglass does not seem to be critical.

The fiber suppresses cracking if the fiber volume is at least about 30%.The thermoplastic ensures a resin rich weld.

Described in greater detail in U.S. patent application Ser. No.08/469,604, which we incorporate by reference, “smart” susceptors aremagnetic alloys that have high magnetic permeability's but that alsohave their magnetic permeabilities fall to unity at their Curietemperature. At the Curie temperature, then, the susceptors becomeinefficient heaters. The alloys are selected to have Curie points closeto the process temperature of welding and have low thermal expansioncoefficients to match composites. The preferable alloys for thisapplication are in a composition range of from 36% Ni to 44% Ni in Fe.Additional alloying elements such as Al, Cb and Ti allow these lowexpansion iron-nickel alloys to be age hardened and add to the cap/skinpulloff strength.

The need for a susceptor in the bond line poses many obstacles to thepreparation of quality parts. The metal which is used because of itshigh susceptibility differs markedly in physical properties from theresin or fiber reinforcement, so dealing with it becomes a significantissue. A reinforced susceptor, which is described in U.S. patentapplication Ser. No. 08/469,986, overcomes problems with conventionalsusceptors by including delicate metal foils (0.10-0.20 inchwide×0.005-0.010 inch thick; preferably 0.10×0.007 inch) in tandem withthe warp fibers of the woven reinforcement fabric. The woven arrangementholds the foils in place longitudinally in the fabric in electricalisolation from each other, yet substantially covering the entire widthof the weld surface. This arrangement still allows adequate space forthe flow and fusion of the thermoplastic resin. Furthermore, in the bondline, the resin can contact, wet, and bond with the reinforcing fiberrather than being presented with the resin-philic metal of theconventional systems. There will be a resin-fiber interface with onlyshort runs of a resin-metal interface. The short runs are the length ofthe diameter of two weave fibers plus the spatial gap between the weavefibers, which is quite small. Thus, the metal is shielded within thefabric and a better bond results. In this woven arrangement the foil canassume readily the contour of the reinforcement. Finally, thearrangement permits efficient heat transfer from the foil to the resinin the spatial region where the bond will form.

Conventional susceptors are essentially planar (X-Y) metal sheets orlaminates of planar films. Welds that embed these susceptors lackreinforcement in the Z-plane, but welds can include such reinforcement(with corresponding improvement in the pulloff strength) if theyincorporate a barbed susceptor of U.S. patent application Ser. No.08/486,560. A barbed susceptor typically uses a Fe—Ni alloy susceptorthat is formed to include barbed, Z-pin reinforcement to provideimproved pulloff strength. The alloy chosen for this susceptor has acoefficient of thermal expansion(CTE) that essentially matches the CTEof the composite and a Curie temperature of about 700° F. (370° C.),which is essentially ideal for thermoplastic welding of resins likeKIIIA polyimide since it is slightly above the resin's melt temperature.For this application, an alloy of 42% Ni—58% Fe including γ′strengthening elements of Al, Ti and Cb yields both low CTE and highstrength. The susceptor is preferably made by laser cutting a foil ofthe material to form barbed tabs and pushing the cut tabs alternately upand down to give the susceptor a three dimensional character.Alternatively a woven wire mesh may be used in this application withalternating wires extending in the Z direction. The thermoplastic resincures or consolidates around the barbs during the welding process whichprovides the pulloff strength improvement.

The barbed susceptor of U.S. patent application Ser. No. 08/486,560usually is fabricated from an age-hardened Invar foil having a thicknessof from 0.003-0.010 inch (0.075-0.25 mm). It may be made from othermaterials having good electrical conductivity and high magneticpermeability. The susceptor may have a pattern of openings made byforming barbs in the Z-axis by folding prongs out of the X-Y plane. Theresult is a susceptor that resembles barbed wire. Each prong of thesusceptor might also be barbed like a fishhook. Such barbs are readilyformed simply by scoring the prong with a cut that starts relativelycloser to the body of the susceptor and extends into the prong at anangle running from the surface toward the tip. This Invar susceptor is“smart”, and helps to avoid excessive heating, because of its Curiepoint.

The barbed susceptor may also have a pattern of openings in the X-Yplane with uniform line widths of about 7 mils (0.18 mm) to define theperipheries of the diamond, as the other susceptors do, so that a fusionbond can occur through the susceptor. Of course, the openings can haveshapes other than diamonds. The diamonds are easy to form by etching,stamping, or expanding and provide a convenient mechanism to control thelongitudinal and transverse impedance, as described in Boeing's otherpatent applications. The diamonds can have L/W ratios less than or equalto 1.0 in the selvaged susceptor where Boeing was interested ininfluencing the eddy currents to run transversely into the solid edgestrips. Other shapes can be used for the openings to create a foil thathas a uniform impedance or whatever desired ratio in the longitudinaland transverse directions.

The barbed susceptor might be a “reinforced” multistrip susceptorsimilar to that described in U.S. patent application Ser. No. 08/469,986with the strips being periodically cut to create Z-plane barbs. Thismultistrip concept may actually be best suited for resistance weldinglike that described in U.S. patent application Ser. No. 08/470,168 orheating in our induction solenoid coil heating workcell of U.S. Pat.Nos. 5,624,594 or 5,641,422, because these two processes induce currentsthat run longitudinally through the susceptor. The multistrip susceptorhas low longitudinal impedance.

Welding researchers have devoted significant effort to develop inductorand susceptor systems to optimize the heating of the bond line in thewelded thermoplastic assemblies. Another hurdle remaining to perfect thewelding process to the point of practical utility for producing largescale aerospace-quality structures in a production environment is theaspect of the process dealing with the control of the surface contact ofthe faying surfaces. This aspect of thermoplastic welding controls thetiming, intensity, and schedule of heat application. The material at thefaying surfaces is brought to and maintained within the propertemperature range for the requisite amount of time for an adequate bondto form. Then, intimate contact is maintained while the melted orsoftened material hardens in its bonded condition.

Large scale parts, such as wing spars and ribs, and the wing skins thatare bonded to the spars and ribs, are typically on the order of 20-30feet long at present, and potentially, can be several hundred feet inlength when the thermoplastic welding process is perfected forcommercial transport aircraft. Parts of this magnitude are difficult toproduce with perfect flatness. Instead, the typical part will havevarious combinations of surface deviations from perfect flatness,including large scale waviness in the direction of the major lengthdimension, twist about the longitudinal axis, dishing or sagging of “I”beam flanges, and small scale surface defects such as asperities anddepressions. These irregularities interfere with full surface areacontact between the faying surfaces of the two parts and can result insurface contact only at a few “high points” across the intended bondline. Additional surface contact can be achieved by applying pressure tothe parts to force the faying surfaces into contact, but full intimatecontact is difficult or impossible to achieve in this way. Applying heatto the interface by electrically heating the susceptor in connectionwith pressure on the parts flattens the irregularities when the resinmelts. Additional time is needed after flattening to achieve fullintimate contact. Extended use of heat and pressure may be excessive,however, and may result in deformation of the top part. When the overalltemperature of the “I” beam flange is raised to the softening point, itwill begin to yield or sag under the application of the pressure neededto achieve a good bond. If sagging occurs the necessary pressure will belost and so will the final product configuration.

Boeing's multipass thermoplastic welding process described in U.S. Pat.No. 5,486,684 enables a moving coil welding process to producecontinuous or nearly continuous fusion bonds over the full area of thebond line to yield high strength welds reliably, repeatably, and withconsistent quality. This process produces improved low cost, highstrength composite assemblies of large scale parts, fusion bondedtogether with consistent quality. It applies heat according to aschedule that melts the resin at the faying surfaces yet maintains theoverall temperature of the structure within the limit in which itretains its high strength. It avoids sagging and, so, does not requireinternal tooling to support the structure against sagging whichotherwise could occur above the high strength temperature limit. Theprocess also produces nearly complete bond line area fusion on standardproduction composite material parts having the usual surfaceimperfections and deviations from perfect flatness. The welding processeliminates fasteners and the expense of drilling holes, inspecting theholes and the fasteners, inspecting the fasteners after installation,sealing between the parts and around the fastener and the holes;reducing mismatch of materials; and arcing from the fasteners.

In the process, an induction coil is passed multiple times over a bondline while applying pressure at least in the region of the coil to theassembled components to be welded and maintaining the pressure until theresin hardens. The resin at the bond line is heated to the softening ormelting temperature with each pass of the induction coil and pressure isexerted to flow the softened/melted resin in the bond line and to reducethe thickness of the bond line while improving the intimacy of thefaying surface contact with each pass. Multiple passes then complete thecontinuity of the bond. The total time at the softened or meltedcondition of the thermoplastic in the faying surfaces is sufficient toattain deep inter diffusion of the polymer chains in the materials ofthe two faying surfaces throughout the entire length and area of thebond line. Doing so, produces a bond line of improved strength andintegrity in the completed part. Because the total time of the fayingsurfaces at its softening temperature is separated into severalsegments, heat in the interface dissipates between passes so that eachsubsequent pass reheats the resin at the faying surfaces but does notraise the temperature of the entire structure to the degree at which itloses its strength and begins to sag. The desired shape and size of thefinal assembly is maintained.

Another moving coil welding operation seeks to apply a substantiallyconstant and uniform pressure on the entire bond line throughout thewelding operation. As described in U.S. patent application Ser. No.08/367,557, such a welding operation, which Boeing calls “fluidtooling,” includes an elongated vessel made of fluid impervious flexiblematerial. The vessel has an elongated axis and an open end at each axialend of the vessel, and has a cross sectional dimension sized toaccommodate the coil. Each axial end of the vessel is closed and sealedby an end closure. At least one of the end closures is removable forinsertion of the coil into the vessel. A linear guide in the vesselextends axially for substantially the full length of the vessel andguides the coil for movement axially through the vessel. Power leads areconnected to the coil and extend through a pass-through in one endclosure to connect the coil to a source of high frequency electricalpower to energize the coil to produce an alternating magnetic field. Amotive system is provided for moving the coil axially along the vesselover the bond line at a controlled speed. The motive system generallyincludes a pair of magnets guided along opposite sides of the vessel andmagnetically coupled to a ferromagnetic mass connected to the coil. Themagnets are moved along their guides and pull the coil attached to theferromagnetic mass inside the vessel. A backup structure exerts adownward force along the top of the vessel, pressurizing fluid sealed inthe vessel and distributing the pressure uniformly over the top surfaceof the top part to press the top part against the bottom part andfacilitate fusion bonding of the thermoplastic in the faying surfaces ofthe interface.

b. Fixed Coil Induction Welding

Boeing has also experimented with thermoplastic welding using itsinduction heating workcell, and, of course, discovered that the processdiffers from the moving coil processes because of the coil design andresulting magnetic field. The fixed coil workcell presents promise forwelding at faster cycle times than the moving coil processes because itcan heat multiple susceptors simultaneously. The fixed coil can reduceoperations to minutes where the moving coil takes hours. The keys to theprocess, however, are achieving controllable temperatures at the bondline in a reliable and reproducible process that assures quality weldsof high bond strength. Boeing's fixed coil induces currents to flow inthe susceptor differently from the moving coils and covers a largerarea. Nevertheless, Boeing has developed processing parameters thatpermit welding with its induction heating workcell using a susceptor atthe bond line. The fixed coil process is described in greater detail inU.S. Pat. No. 5,624,594, which we incorporate by reference.

Another advantage with the fixed coil process is that welding can occurusing the same tooling and processing equipment used to consolidate theskin, thereby greatly reducing tooling costs. Finally, the fixed coilheats the entire bond line at one time to eliminate the need for shimsthat are currently used with the moving coil. Boeing can control thetemperature and protect against overheating by using its “smart”susceptors as a retort or as the bond line susceptor material or both.

c. Temperature Monitoring

In U.S. patent application Ser. No. 08/548,823 Boeing describes a systemfor thermoplastic welding to monitor the bond line temperature in realtime allowing detection of the onset of flow of the thermoplastic resin.The system permits guidance control of the induction head to adjust itspower, speed, or motion in response to the measured temperature.Basically, Boeing embeds at least one multinode thermocouple within theweld near the bond line in a layer adjacent the susceptor to measure thetemperature under the moving coil.

The thermocouple is made by twisting the wires together or in a zig-zagfashion to form periodic nodes along the bond line. A single wirethermocouple configuration using constantan wire and using the coppersusceptor as the second conductor also possible. The spacing of thenodes depend on the desired resolution, but, should be about 0.2 inch orso apart.

The thermocouple will be an open circuit prior to the onset ofthermoplastic flow, and will not have a voltage output. At the onset offlow, the two thermocouple wires short and produce a thermoelectricvoltage proportional to the temperature of the thermocouple junction.The thermocouple will read the temperature directly under the inductionhead, that being the hottest junction and also the one that is closestto the monitor input. The multinode thermocouple behaves like a seriesof parallel batteries. The node closest to the monitor produce thehighest voltage amplitude because it directly in the hot zone. The samenode also acts as a short to any other voltages produced by thermocouplenodes further away from the monitor. Each consecutive junction shortsthe potential generated by the preceding node. If the node contactresistance is high there may be a small error.

As described in U.S. patent application Ser. No. 08/548,823 Boeingwelded a test panel with a sliding junction (multinode) thermocouple inthe bond line. The thermocouple was made with two bare Chromel/aluminel,AWG #36 wires and wound in a zig-zag way on a piece of thermoplasticresin or was encapsulated with the resin. The thermocouple was locatedhalf way between the center of the bond line and the edge. Boeing alsowelded a second test panel with two multinode thermocouples near edgesof the susceptor in the bond line. The thermocouples were located halfway between the center and the edge on each side of the bond line, withnodes spaced one inch apart. The output of the two thermocouples trackedwithin 25° F.

By locating the thermocouples on the outer edges of the bond line, thevoltages generated by the two thermocouples produce a guidance controlfunction formed by combining the two thermocouple outputs with adifferential amplifier bridge circuit. When the coil moves off center,it will produce uneven heating across the bond line. This heating willresult in a differential thermocouple output signal used to restore thecoil to the center of the susceptor, and, thereby, restore uniformheating across the bond line. Nevertheless, there also remains problemswith the accuracy of positioning in the assembly, with shorting, andwith reproducibility in what currently is a task requiring relativelyhigh skill.

A drawback to this multinode thermocouple method of process monitoringand control for induction welding is that it is intrusive. Thethermocouple wires remain in the bond line. The diameter of thethermocouple wires are as small as 0.001 inch. They should not presentsignificant structural problems. The insulation of the thermocouple wireshould be the same thermoplastic resin as that being welded and shouldnot have any adverse effect on the structural properties of the bond.

In U.S. Pat. No. 5,573,613, Boeing also described a method fordetermining the susceptor temperature by measuring the change inimpedance of the induction coil. As the susceptor heats, its electricalresistance changes as a function of the thermal coefficient ofresistance (TCR) of the susceptor material, and that change is reflectedback as a change in the drive coil impedance. An electrical circuitsenses the varying impedance/resistance and converts that change into achange of temperature on a temperature display, or into a signal toadjust the power to the coil or the speed of travel of the coil alongthe bond line. The sensing circuit includes a high power bridge with asensitive null arm to sense changes in the susceptor impedance due totemperature changes.

A simple L-R bridge detects the changing resistance of the susceptor asits temperature changes during inductive heating. The bridge includes ahigh-power transformer of about 500 watts operating at about 35-55 kHzconnected across a pair of series-connected inductors L₁ and L₂ and apair of series-connected resistors R₁ and R₂. Both series-connectedpairs are connected to each other in parallel and in parallel with thetransformer. A shunt with a voltage sensor (such as a voltmeter or anoscilloscope) is connected between the two resistors and the twoinductors. The two sides of the bridge are asymmetric by at least 2:1 toput most of the power in the bond line for the sake of efficiency, sincepower dissipated in the reference side of the bridge is wasted. The twocoils L₁ and L₂ are designed to track fairly closely so that theirinductances and Q's (i.e. the dimensionless power ratio of stored todissipated power) vary consistently with frequency. One of inductors L₁or L₂ is the moving coil to transfer energy to the susceptor.

The bridge signal is used to control the welding process interactivelyby adjusting the power to the coil in a closed loop RF heating controlcircuit, or by adjusting the speed of travel of the coil over the bondline, or both, so as to maintain the melt pool temperature within thedesired range of optimum processing temperature, that is, 620±25° F. inthe case of the DuPont Avamid KIIIB polyimide. The signal is conditionedin a suitable conditioning circuit, which would depend on the voltagesensor used and could produce a digital signal to the power amplifier toturn the amplifier up or down, in the nature of a thermostat control,whenever the melt pool temperature drops below or exceeds the optimaltemperature range. Preferably, the signal conditioner circuit produces asignal proportional to the voltage sensor signal to adjust the power tothe work coil up or down from a predetermined average power level knownto maintain a steady state temperature in the melt pool at the coilspeed used. Nevertheless, overheating can still be a significantproblem, especially if localized overheating arises from misalignmentbetween the moving coil and the susceptor.

d. Steering the Moving Coil

A nonintrusive system associated with a moving induction coil,particularly one of the type described in U.S. Pat. No. 5,313,037,self-steers the coil over the susceptor to avoid excessive, damagingoverheating that otherwise might occur because of misalignment betweenthe coil and the susceptor. Alternately, the system can sense themisalignment by the aberration in the magnetic field and can create acompensating “hot spot” with a differential, parasitic, secondary coil.

In U.S. patent application Ser. No. 08/565,566, when there is amisalignment between the primary coil of the induction head and thesusceptor, the self-steering system produces a guiding command with asecondary coil to return the primary coil to the centerline.Alternately, the system can use a differential, parasitic secondary coilto compensate for the misalignment and to achieve better temperatureuniformity in the bond line by adjusting the magnetic field. Toaccomplish these features, Boeing uses two, peripheral coils connectedin differential mode to produce a null (i.e., no differential voltage)when the coil is centered over the susceptor. Doing so, Boeing tips thecoils at 45° on the sides of the cup coil of U.S. Pat. No. 5,313,037.

A compensating secondary coil can be located in the centerline of thedrive coil. This secondary coil has a “lazy 8” design and produces nomeasurable effect when inserted between the primary coil and the partsassembled for welding, unless the coil and susceptor are misaligned.When there is misalignment, the “lazy 8” forms a compensatory “hot spot”on the side of the susceptor that would otherwise be cool because of themisalignment. Compensation occurs provided that the “lazy 8” has a totalresistance lower than the eddy, but the effect does not fully compensatefor the offset.

Evaluating the Quality and Integrity of the Thermoplastic Weld

Turning now to FIG. 1, a thermoplastic welding head 10 that includesleading and trailing pneumatic pressure pads and a primary inductioncoil 25 disposed between the pads is supported on tooling headers 12over thermoplastic composite parts to be fusion bonded together. Theparts, in this example, include a thermoplastic spar 14 and athermoplastic wing skin 16, only a small section of which is shown inFIG. 1. The spar 14 is in the form of an “I” beam having a top cap 18, abottom cap 20, and a connecting web 22. The spar 14 extends lengthwiseof the wing of the airplane for which the parts are being assembled, andthe wing skin is bonded over the full length and surface area of thespar cap 18 with sufficient strength to resist the tensile and peelingforces the wing will experience in flight. The apparatus shown is morefully described in U.S. Pat. No. 5,556,565. The beams might be allcomposite construction or a hybrid metal webbed composite capped beam asdescribed in U.S. Pat. No. 5,556,565. We could also join thermoset skinsand spars with a hot melt thermoplastic adhesive.

A copper mesh susceptor 32 (i.e., a metal foil 702 susceptible toinduction heating encapsulated in a thermoplastic resin 704, FIG. 7) isinserted between the spar cap 18 and the wing skin 16. Typically theencapsulating resin is the same or a slightly lower melting temperatureformulation of the same thermoplastic resin of the spar cap 18 and thelower faying surface of the wing skin 16.

The welding head 10 can be any moving coil apparatus that is capable ofapplying pressure during induction heating of the bond line to promotefusion and after heating for a period sufficient for the resin to cooland harden in its bonded condition. Suitable welding heads are disclosedin U.S. Pat. Nos. 5,635,094; 5,444,220; and 5,313,037. A preferredwelding apparatus includes an induction coil 25 for inducing eddycurrents in the susceptor 32. The eddy currents heat the susceptor byelectrical resistance heating and soften or melt the thermoplastic resinin the faying surfaces of the parts so it flows, interdiffuses, andfuses together with softened resin of the wing skin and spar cap uponcooling.

The coil shown in the '037 patent provides zero eddy current at thecenter with the current density increasing toward the edges. Use of atailored susceptor is desirable to counterbalance the nonuniform eddycurrent density that the coil produces from centerline to edge toachieve uniform heating, and such a susceptor is disclosed in U.S. Pat.No. 5,500,511. A selvaged susceptor designed especially for use with theasymmetric induction coil of U.S. Pat. No. 5,444,220 is described inU.S. Pat. No. 5,508,496.

The primary induction coil 25 is mounted in the welding head 10 in thecenter of a lower frame which is pinned to a link connecting the lowerframe to an upper frame. The upper frame is pulled by a motive apparatusincluding a stepper motor driving a drive sprocket and a chain loopthrough a reduction gear unit. A pair of camroll bearings projects fromboth sides of the lower frame into cam grooves milled into the insidesurfaces of the headers to guide and support the lower frame. A similarset of camroll bearings projects outward from the upper frame into astraight cam groove to guide the upper frame as it is pulled by thechain loop from one end of the wing skin to the other.

The process of welding the wing skin to the spar cap begins withassembling the parts together with the susceptor 32 interposed betweenthe faying surfaces of the parts. In the case of a wing box, we attachthe susceptor 32 to the outer surfaces of the spar caps 18 and 20 andthen sandwich the spars between the upper and lower wing skins 16. Theparts are held in position and squeezed together by a force exerted by apair of air bearing pads to which air under pressure is delivered by wayof air lines and distributed to the air bearing pressure pads byseparate air lines. The air to the pads reduces the frictional drag onthe pressure pads on the top surface of the wing skin and helps to coolthe parts after the coil has passed. The induction coil 25 moves alongthe intended bond line over the outer surface of the wing skin ingeneral alignment (±0.125 in) with the susceptors while producing analternating magnetic field which projects through the wing skins andaround the susceptor, generating eddy currents in the susceptor. Theeddy currents induced by the magnetic field are of sufficient amperageto heat the susceptor, raising the temperature of the thermoplasticmaterial in the faying surfaces to its softening or melting temperature.After the first pass of the welding head over each bond line to seal thebox, the process is repeated three or more times, usually increasing thepower to the coil after the second pass and, if desired, increasing thepressure exerted by air cylinders on the pressure pads.

The bond strength improves with multiple passes of the welding head overthe same bond line. Multiple passes of the induction coil serves tocreate the optimal conditions for achieving a fusion bond with thedesired characteristics of continuity over the entire bond line, andsubstantial molecular interdiffusion of the materials in the fayingsurfaces to produce a bond line of high pulloff strength with thecomplete or nearly complete absence of voids, as discussed in U.S. Pat.No. 5,486,684. Welds having higher pulloff strengths use a barbedsusceptor of U.S. patent application Ser. No. 08/486,560 on the bondline.

The mechanisms for achieving a fusion bond include intimate contact and“healing.” Intimate contact of the two faying surfaces is a function offorce exerted on the parts to squeeze them together, andtemperature-dependent viscosity. The force exerted on the parts isdistributed over a certain surface area as interfacial pressure tendingto bring the faying surfaces together. The viscosity of the surfacematerial is manifested by the tendency of high spots in the surface toyield of flow so that low spots in the two surfaces can come together.“Healing” is partly a process in which molten or softened materials flowtogether and blend where they come into contact, and partly a process ofmolecular penetration of the polymer chains in the material of onesurface into the molecular matrix of the material in the other fayingsurface. The average penetration distance of the polymer chains, withoutthe beneficial mixing effect achieved by flowing the materials in thefaying surfaces, increases as a quarter power of time (i.e., t^(0.25)).

Objective and easily made observations of a bond line that areindicative of “healing” of the quality of the bond are reduction in bondline thickness, improved ratio of bonded to unbonded surface area in thebond line (or expressed conversely, a reduction of the amount ofunbonded surface area in the bond line), and improved pass-through of abonding resin through openings in the susceptor.

Irregularities, such as hollows, depressions, and asperities (i.e.,peaks) in the faying surfaces of the parts, and other deviations fromperfect flatness can interfere with and prevent continuous intimatecontact along the full surfaces of the parts where bonding is intended.These deviations from perfect flatness include small scale surfacefeatures such as asperities, depressions or hollows, scratches andbumps, and also large scale features such as waviness in the directionof the major length dimension, twist about the longitudinal axis,dishing or sagging of “I” beam flanges, and warping such as humping orbowing in the longitudinal direction. The structural susceptor isparticularly suited for dealing with these problems.

Boeing's goal is to produce aircraft structure that eliminatesfasteners. Welded structure will be far less expensive because weldingeliminates the labor to drill holes accurately and to inspect thefasteners after installation. We also will avoid other problems thatfasteners introduce, such as sealing around the fastener and the holes,mismatch of materials, and arcing from the fasteners. To replace thefasteners, however, requires confidence that the welds are uniform andconsistent. A failure at any weak point in the weld could lead tocatastrophic unzipping of the entire welded structure. One of the mostimportant problems with quality welding is temperature uniformity alongthe bond line to achieve uniform and complete melt and cure of theresin. Being a “smart” susceptor, our barbed susceptor has a Curietemperature slightly higher than the welding temperature (i.e., about700° F.) so the possibility of disastrous overheating is reduced. Thepresent invention is a reliable method to test the weld quality able todistinguish low strength, inadequate bonds from well bonded welds.

Boeing embeds the foil in the resin to simplify the welding process.Making a foil/resin tape eliminates the steps of applying separatelayers of resin between the respective elements in acomposite-susceptor-composite assembly. It also ensures that there willalways be adequate resin proximate the susceptor and essentially uniformresin thickness across the welding bond line. The typical tape is about2-4 inches wide with KIIIA Avimid resin (an aromatic polyimide),although the resin can be PEEK, PEKK, PES, PEK, ULTEM, or any otherthermoplastic. The resin must be compatible with the matrix resin in thecomposite and generally is the same resin as the matrix resin whenwelding thermoplastic composites. For the “welding” analog for thermosetcomposites, the resin will likely be a comparable thermoplasticformulation of the matrix resin in the composites or a compatible resin.

As shown in FIG. 2, the transmitter-receiver 200 of the presentinvention is similar in overall design to the cup coil of U.S. Pat. No.5,313,037. The transmitter-receiver, nonetheless, is adapted for thenondestructive evaluation (NDE) method of the present invention toproduce an electromagnetic pulse to vibrate the susceptor and to receivethe returning acoustic signal representing the susceptor vibration. Thetransmitter-receiver 200 has a housing 205 which contains the cup(pancake) coil 210 similar to that described in U.S. Pat. No. 5,313,037around a central pole 215. Unlike the patented cup coil, however, thecentral pole 215 in the present invention carries a shielded acousticemission transducer (receiver) 220 at the center of the pole. Thetransmitter-receiver 200 may also include active cooling plumbing forcirculating cooling water or other suitable coolant around the pancakecoil 210 during its operation, analogous to the cooling circuit forBoeing's induction coil. Applicants generally do not include thiscooling plumbing.

FIG. 2 also illustrates that the pancake coil 210 is connected to a 240μF capacitor bank 225 and high voltage D.C. power supply 230 so that anelectromagnetic pulse of predetermined characteristics (i.e., time,energy, frequency, and amplitude) can be introduced to the coil 210 byactivating the switch 235. The power supply typically supplies power inthe range from 0-10 kV, and we prefer 500 V. The pulse to the coil hasthe characteristics generally shown in FIG. 4 with a duration of about0-10 msec at 500±0.1-5.0 V.

The receiver circuitry is also shown in FIG. 2, and includes adigitizing oscilloscope 240 connected to the acoustic emission receiver220 for viewing the acoustic signal representing the susceptorvibration; a computer processor 245 for transforming the acoustic signalfrom the time domain to the frequency domain or to provide othersuitable signal processing to allow discrimination of the weld quality;and a printer 250 for plotting the various evaluation results. Thesensitivity of the receiver is a function of the pulse current, thedistance through the composite to the susceptor, the thickness of thecomposite, and the conductivity of the susceptor and composite. Thecurrent and distance are factors because they represent the strength ofthe induced magnetic field reaching the susceptor. The compositethickness is a factor because the returning acoustic signal must travelthrough the composite. The conductivities are also related to themagnetic field strength at the susceptor. We have not discovered anydifferences in performance because of the use of different resins in thecomposite or the bond.

As shown in FIG. 2, the transmitter-receiver 200 is positioned over thewelded assembly 300 (such as a composite skin to composite spar weld) topulse the susceptor 32 and to receive the resulting acoustic signal thatthe vibrating susceptor creates. The acoustic emission receiver 220 isgenerally located above the centerline of the susceptor 32 inside(substantially concentric with) or outside the windings of a pancakecoil 210. The AE receiver might also be located on the opposite face ofthe assembly if access to both sides of the assembly is possible. Inmost of our tests we used this alternate “through assembly” arrangement,but we prefer the transmitter-receiver arrangement shown in FIG. 2 forassessing authentic aerospace structure where blindside access generallyis impractical or unavailable. An EM pulse from the pancake coil 210penetrates through the composite skin 255 and into the weld 260 where itinduces eddy currents in the susceptor 32. The pulse may involve thelower composite spar or web cap 265 without significant interaction withthe composite.

To conduct a test, we step the transmitter-receiver incrementally alongthe bond line limited only by the time required to recharge thecapacitor bank, which is quite fast. For an automated inspection, wewould couple the transmitter-receiver to a stepper motor or othersuitable motive means to convey the transmitter-receiver incrementallyover the bond line. At each pulse, the transmitter-receiver isstationary.

We believe that our test method should work with continuous or segmentedsusceptors and with dispersed microparticles at the bond line, althoughour tests have focused on narrow (2-3 inch) and wide (4-5 inch) coppersusceptors having an expanded diamond pattern or etched square patternof openings, like those described in Boeing's patents. We have nottested a bond line reinforced with Z-pins, but we expect our method towork there as well. The Z-pins may alter the characteristic returnsignal, however, by modulating the susceptor's vibrational modes.

When pulsed, the susceptor 32 experiences stresses that translate intoan acoustic signal 400 representative of the susceptor vibration thatresults because of the stresses. FIG. 3 shows the theoretical stress onthe susceptor created by a 20 mV, 15.6 kAmp pulse of about 0.0001 secduration. This stress produces an acoustic signal from the compressionand tension of the weld resin that propagates through the assembly'scomposites to the AE receiver 220. FIG. 4 shows a typical analogacoustic signal in the time domain (i.e., amplitude v. time), which wetransform the acoustic return signal using a suitable Fourier transformalgorithm or other suitable transform to the frequency domain (i.e.,amplitude v. frequency). Our signal processing is applicable to analogor digital representations of the acoustic vibration, although analogprocessing probably is simpler. In the frequency domain 500, we are ableto discriminate between welds of adequate strength and those ofdangerously low or no strength. In particular, FIG. 5 shows the typicalspectral response for a weld having adequate strength while FIG. 6 showsthe spectral response for a weld having inadequate strength. Thesignificant characteristic in the spectral response between an adequateweld and an inadequate, low strength weld is the presence of lowfrequency peaks 605 in the spectral response in the 1-3 kHz range forthe inadequate welds. The precise location of this low frequency signaldepends upon the geometry of the part under test and the susceptor, but,for all configurations, we have been able to distinguish quality weldsfrom low strength welds or bonds by finding this low frequency peak inthe return of the low strength bonds.

FIG. 8 shows the relationship of stress in the susceptor as a functionof position displaced from the centerline when a pulse is transmittedwith our pancake coil transmitter and the return signal is received atthe center of the coil. The stress is bimodal and is symmetrical aboutthe central pole of the coil and centerline of the susceptor. That is,δ=0.

Alignment between the transmitter-receiver and susceptor does not appearto be a critical concern, which makes our method easier to use. Weachieve the best results, however, when the receiver is about 1 inchlaterally from the transmitter. The vibrating susceptor creates adispersive, global acoustic signal in the composite. The pulse, being soshort in duration, apparently does not heat the susceptor or bond linesignificantly.

The area under the vibration signal curve for frequencies up to 1 kHzprovides a convenient correlation for the strength of the bond. The bondstrength is inversely proportional to the area. FIG. 9A-D show bonds offour different strengths, the frequency response curve up to 1 kHz, andthe area under the frequency response curve. The correlation for anarrow susceptor I-beam weld. Values of strength are normalized relativeto a reference strength and the normalized plot of FIG. 10 shows asubstantially linear degradation of relative bond strength as the areaunder the response curve increases.

Narrow susceptors exhibit essentially Mode 1 vibrations. Widersusceptors may vibrate differently, so the bond strength v. areacorrelation may not hold for wider susceptors.

While we have described preferred embodiments, those skilled in the artwill readily recognize alterations, variations, and modifications whichmight be made without departing from the inventive concept. Therefore,interpret the claims liberally with the support of the full range ofequivalents known to those of ordinary skill based upon thisdescription. The examples are given to illustrate the invention and notintended to limit it. Accordingly, limit the claims only as necessary inview of the pertinent prior art.

We claim:
 1. A thermoplastic rewelding method for forming athermoplastic weld between at least two, fiber-reinforced compositelaminates containing thermoplastic resin and repairing welded areas ofinadequate strength, comprising the steps of: (a) assembling at leasttwo, fiber-reinforced, composite laminates containing thermoplasticresin to define a bond line along faying surfaces of the laminates; (b)positioning a metal mesh susceptor along the bond line; (c) heating thelaminates along the bond line to form a thermoplastic weld between thelaminates by inductively heating the metal mesh susceptor to melt,intermix, and fuse the resin around the metal mesh susceptor to weld thelaminates together; (d) nondestructively evaluating the thermoplasticweld quality by analyzing acoustic signals generated by electromagneticpulses absorbed in the metal mesh susceptor; and (e) rewelding in atleast those regions of the thermoplastic weld found to have inadequatestrength by reheating the metal mesh susceptor.